explain why the percentage of poeple with $300,000 or more increase so substantially across the age groups

Because in accumulation of wealth, older people have an advantage.

Explanation:

Older people tend to have more money simply because:

1. they have had longer careers and hence are expected to have better salaries with better positions

2. they have had more time to save up capital and invest.

Every age group chronologically is more able to collect money in their working years. Young people starting their careers are less likely to be able to accumulate wealth to have a worth that much.